Yeah, after I posted my reply it dawned on me that was odd that didn't happen with Ext 3.1.

Not sure what to suggest really.
Personally I'm fed up with having to deal with IE in all its forms, but sadly not really a lot we can do about it. It's what big corps tend to use, or people who don't know or care about their browser. Sadly that amounts to a lot of people...

I think you'll probably find that the memory is being marked as freeable, but it's not being reclaimed by the browser.
Perhaps try on a machine with less memory so that the memory management is more aggressive?
Have you got access to IE10 yet, might be worth a try?

I am in the middle of porting our single page application from Ext 3.4 to 4.2. I have both sites running side by side ( on different urls) and I can confirm that the leak is not happening on Ext 3.4 (or at least it is not as bad as 4.2). I can easily hit 200M with 6 refreshes on IE9.

Did you find a solution? The doc tags did not make a difference for me.